The Kyn-containing peptide FP-Kyn-L(NH2) is an unusual minor component of the skin peptide profile of the Australian red tree frog Litoria rubella collected from an area within a 20 kilometre radius of Alice Springs in central Australia. The structure was determined by electrospray mass spectrometry and synthesis. The major component of the skin secretion is the analogous tryptophyllin peptide FPWL(NH2). Both peptides show opioid activity at 10–7M, and are likely to act via the μ opioid receptor.